With automotive designs tending to lowered hood lines for better appearance of the front end of the vehicle and also better aerodynamic performance the need for smaller headlamp packaging is apparent. Smaller headlamp packaging requires smaller light bulbs which operate at higher temperatures, up to 700 degrees centigrade, due to their smaller size. These light bulbs, which are typically of the double-ended type but can be of the single-ended type, are made from quartz material. The light bulb is typically fixed onto a plastic base inside the headlamp assembly. The plastic base and the bulb mounted thereon forms a replaceable bulb. In order for the lamp to be acceptable to the average consumer, it must be cost competitive relative to other similar headlamps and must also be reliable. Therefore the replacement bulb must be of a construction with fewer parts for ease of assembly, quality and cost reasons. Proper alignment and stability of the light bulb inside the headlamp assembly are important requirements for the performance of the headlamp. The light bulb must be aligned in a specific position with respect to the plastic base. Once aligned and precisely locked in the appropriate position with respect to the plastic base, the bulb must remain in that aligned position during use, for acceptable performance of the headlamp. In fact, the lamp must conform to Federal Motor Vehicle Safety Standard 108 which defines a deflection test for replaceable headlamp bulbs. The deflection test requires that the light source must be rigidly mounted in the lamp fixture in such a manner that the bulb does not permanently deflect beyond a specified distance of 0.005 inches when a 4 pound force is applied to the bulb perpendicular to its longitudinal axis.
A gimbal device is one of the mechanisms typically used to align the light source with respect to the plastic base. The gimbal, which serves as a holder for the bulb may also be required to insulate the plastic base from the heated bulb. The gimbal is generally contained within a cylindrical opening in the plastic base from which the light bulb can be aligned, after which the gimbal is attached to the plastic base by either thermal or mechanical means so as to secure the light bulb in the aligned position.
U.S. Pat. No. 4,569,006 describes one type of gimbal device designed to align and precisely lock the light source in the aligned position while providing a system that is readily adaptable to mass production for cost reasons. While serving its intended function, this gimbal design has the disadvantage that it comprises three distinct pieces and so does not lend itself to mass production as readily as a construction with fewer pieces. In addition, the use of multiple parts/processes invariably has a negative quality impact resulting in yet additional quality costs.
U.S. Pat. No. 4,795,939 describes a gimbal device comprising two pieces, a ceramic member used as a bulb holder for a single ended quartz bulb and a plastic member that joins the ceramic bulb holder to the plastic base. The plastic member contains a groove onto which a metallic ring is fitted. The ring-fitted end of the plastic member is attached to a cylindrical opening of the plastic base by means of radio-frequency (RF) heating of the metallic ring to form a plastic-to-plastic weld. The two-piece gimbal does not easily lend itself to mass production due to the multiple parts of the gimbal.
From the standpoint of cost, quality, and ease of mass production it would be advantageous to eliminate the intermediate plastic member that connects the plastic base to the ceramic bulb holder. In this case a precise circumferential groove must be made onto the surface of the ceramic holder in order to contain the metallic ring which is heated to cause the ceramic member to be attached to the plastic base using RF energy. In order for this RF thermal attachment method to form a rigid joint, the ceramic must either deform or melt with the application of RF energy or the groove must be precisely sized to the dimensions and shape of the metallic ring.
However ceramics will not soften or melt, so that any flowing which serves to interlock the ceramic and the plastic must be done by the plastic base only. While the plastic base material could melt, there is no guarantee that the plastic base will flow into the ceramic groove and act to stabilize the joint. If the gimbal material will not flow, the joint may be stabilized by manufacturing the gimbal to such tight tolerances that the groove in the gimbal surface is very close-fitting to the metal ring contained in the groove. In this case, the joint is formed by virtue of the protrusion of the metallic ring into the softened cylindrical portion of the plastic base and the melting and flowing of the plastic around the metal. The joint is rigidified by the locking effect of the ring, protruding into the plastic wall while still within the walls of the groove.
This type of joint is dependent on the close-fitting nature of the ring and the groove. If there is any gap between the groove and the sides of the ring namely the tolerance gap, the gimbal may rock slightly, though still locked in the base. The rocking will allow the bulb to move outside the deformation limit of the federal specifications. Due to their nature, ceramic materials can not be molded to tight tolerances required for this type of fit. Ring sizing tolerance could also be tightly controlled for close-fitting of the ring into the groove, but this would be a costly option. The tapered side walls of the groove allow the tolerance gap to increase since the ring expands radially outwards as it melts into the plastic. Thus it is apparent that elimination of the tolerance gap would provide for a tighter more reliable joint. In addition, it would be particularly advantageous from a cost and quality standpoint to achieve such a tighter more reliable joint without the need to maintain a tight ring sizing tolerance.
It is the objective of this invention to provide a one-piece gimbal device with a superior gimbal device/plastic base joint for improved reliability in firmly supporting the light bulb: that contains the fewest parts possible for ease of assembly; that can be manufactured to tight tolerances to produce a high quality part; that is adaptable to mass production at a reasonable cost; and one in which the ring sizing tolerance is not critical for obtaining a rigid gimbal device/plastic joint.